The angle at which a ray strikes a surface is called the angle of incidence. It is measured as the angle between the incoming ray and the normal (a line perpendicular to the surface) at the point of contact.

Here's a breakdown:

* Ray: A line representing the path of light.

* Surface: The boundary between two mediums (e.g., air and water, air and glass).

* Normal: An imaginary line perpendicular to the surface at the point where the ray hits.

* Angle of incidence: The angle between the incoming ray and the normal.

Let me know if you'd like a visual representation or want to discuss how the angle of incidence relates to reflection and refraction!